It is well known that an aromatic polyimide having excellent heat resistance can be obtained by the reaction between an aromatic tetracarboxylic dianhydride and an aromatic diamine as described in C.E. Sroog, Journal of Polymer Science, Macromolecular Review, Vol. 11, page 161, (1976). However, the conventionally proposed aromatic polyimides have been limited in their application due to difficulty in melt-molding.
In order to improve melt-moldability, aromatic polyetherimides prepared by using an aryloxy acid dianhydride as acid dianhydride component have been studied, as disclosed, e.g., in Japanese Patent Publication Nos. 20966/82 and 20967/82, and have been put into the market under the trademark "ULTEM.RTM." from General Electric Company. The aromatic polyetherimides of this type are excellent in melt-moldability (e.g., injection moldability and extrusion moldability) but, in turn, inferior in heat resistance and solvent resistance as compared with the conventional aromatic polyimides.
On the other hand, aromatic polyimides having melt-moldability without suffering great reduction of heat resistance have been reported, such as an aromatic polythioetherimide obtained by reacting an aromatic diamine having a (thio)ether linkage and pyromellitic dianhydride as disclosed in Japanese Laid-Open Patent Application Nos. 170122/84 and 250031/86, and a polyimidosulfone as disclosed in U.S. Pat. No. 4,398,021. They, however, are still unsatisfactory for practical application in the field of engineering and electronics where a balance between heat resistance and mechanical properties is required.
In order to eliminate these disadvantages of aromatic polyimides, Japanese Laid-Open Patent Application No. 15228/87 proposes an aromatic polythioetherimide obtained by the reaction between an aromatic diamine having a thioether linkage and 3,3',4,4'-benzophenonetetracarboxylic dianhydride and/or pyromellitic dianhydride. Although this aromatic polythioetherimide shows an excellent balance between heat resistance and mechanical properties, a further improvement on moldability has been demanded.